The present invention relates generally to air brake systems and more specifically for an air brake system and a relay valve to be used in a vehicle convertible between highway and railroad modes of use.
There has always been a great interest in the combined transportation of highway and rail vehicles. This has generally included the loading of road trailers onto flat bed rail cars which are then transported across the rails and then driven to location away from the rails. Efforts have also been made to equip trailers with road wheels and rail wheels such that the trailer itself forms both a road trailer and a rail car. One such vehicle is described in U.S. Pat. Nos. 4,202,276 and 4,202,277 to Browne et al. The major problem with the prior art system including that of the above-mentioned patents is that the designers have generally designed a brake system using highway technology and criteria which is unacceptable for use in a rail system.
As an example, the system in the aforementioned U.S. Patents use a straight air brake system to operate the brake. In this system the pressure and the brake pipe line are used to control a relay valve which in turn controls the brakes. This style of system had been used extensively in rail vehicles but has been replaced by ABD (air brake diaphragm) valves wherein the valve is responsive to modulation of the brake pipe pressure to produce its own brake control signal. A major advantage of the ABD valves is that they provide better control and quicker response as well as the capability of providing braking control for a further distant or longer length train.
Thus there exists a need for a braking system for a vehicle which is capable of highway and railway operation with a rail fluid braking system meeting the standards of the rail industry.